This proposal is primarily concerned with the electron microscopic examination of the morphology, as well as the temporal and spatial distribution, of the synaptonemal complex in the oocytes of Drosophila melanogaster. A comparison will be made between wild-type females and females carrying meiotic mutants. A subsidiary project is the examination of the effects of recombination-defective meiotic mutants on mitotic recombination. Meiotic mutants, of which over 4 are known in Drosophila, are mutants that result in abnormal recombination, abnormal disjunction, or both. Of particular interest to the present study is a set of mutants (called recombination-defectives) at 14 loci, each of which results in a characteristic decrease in crossing-over on all chromosomes. I have performed serial reconstructions of wild-type oocyte nuclei and have found several features that may be affected by such mutants and therefore provided important insights into the process of recombination of (a) chromocentral organization of pachytene bivalents; (b) differentiation of synaptonemal complex morphology in euchromatin vs. heterochromatin; (c) stage-dependent variation in the length and thickness of the synaptonemal complex; (d) structures associated with the synaptonemal complex which, in pachytene, appear to presage the location of chiasmata. The cytological and genetic effects of these mutants will be compared.